This invention relates generally to seismic exploration of substrata beneath bodies of water and, more particularly, to a marine seismic system for sensing reflected seismic waves from such substrata.
Marine seismic exploration is usually conducted by towing a seismic streamer at a given depth through the ocean or other body of water. The streamer is provided with a plurality of pressure sensors, such as hydrophones, disposed at appropriate intervals along the length thereof. Acoustic wave energy is provided in the vicinity of the cable by an air gun or other suitable means; this wave energy travels downwardly through the earth with a portion of it being reflected upwardly at levels where there is a contrast in the acoustic impedance characteristics of the strata. The pressure sensors detect these primary pressure waves produced in the water by the upwardly traveling seismic reflections and provide electric signals indicative thereof to suitable processing and recording equipment located on the seismic vessel that is towing the streamer. The pressure sensors also receive secondary pressure waves reflected from the surface of the water as a result of the mismatch in acoustical impedance at the air-water interface which may adversely affect the seismic signals. Nearly total cancellation of certain frequencies of the seismic signal may result, since the pressure wave undergoes a 180.degree. phase shift when reflected at the air-water interface. The prior art, such as U.S. Pat. No. 3,290,645, has attempted to overcome this problem by employing both a pressure sensor and a particle velocity sensor. The output signals of the pressure sensor in response to the primary and secondary pressure waves have opposite polarity; whereas, the output signals of the particle velocity sensor have the same polarity for the primary and secondary waves. The prior art combines the pressure signals with the particle velocity signals to cancel the surface reflected wave front or ghost; however, it has been found that the mere combination of a pressure wave signal with a particle velocity signal may severely degrade the signal-to-noise ratio of the lower frequencies in the seismic band. This high noise level in the lower frequencies of the output of the particle velocity sensor is a function of the mounting of the particle velocity sensor and the geometry of the cable. Particle velocity sensors such as those disclosed in U.S. Pat. No. 3,281,768, which consist of either a particle displacement sensor in conjunction with a differentiating circuit or a particle acceleration sensor in conjunction with an integrating circuit may also be subject to the high noise levels.
Therefore, it is an object of the present invention to provide a marine seismic system that eliminates the adverse effects of the reflected, secondary pressure wave on the seismic signal and provides a good signal-to-noise ratio over the seismic band.